Large Wind Turbines

July 28, 2003
Paul Gipe

Return of Two Blades? A new look at an old idea

An edited version of this article appeared in the Summer 2003 (Vol. 16, No. 3) edition of WindStats.

With 270 people--including New Zealand's minister for energy-- attending the dedication of Windflow Technologies prototype wind turbine this past summer, proponents hope a new day may be dawning for two-blade technology. Getting a Kiwi energy minister to attend the dedication of any wind turbine, let alone a novel machine with only two blades is no small feat.

Windflow's designer Geoff Henderson is nothing if not tenacious. He wasn't satisfied to have minister Pete Hodgson shake a few hands, give a speech, and then drive away from the site at Gebbies Pass near Christchurch. Henderson took Hodgson to the top of the 500 kW turbine Where Henderson started the turbine into operation, permitting the minister, to see--and feel--firsthand the power of the wind.

The publicity stunt also enabled Henderson to emphasize the selling points of his all-Kiwi technology. Windflow's "pitch-teeter" coupling provides a "smooth ride at the top, that's the beauty of our system" says Henderson who felt comfortable taking the minister up to the nacelle and running the turbine.

But it's two steps forward and one step back for advocates of two-blade wind turbines. While Henderson was celebrating his long struggle to see his dream come alive, others were wondering if there's was dashed on the baked earth of Southern California when the rotor on the Wind Turbine Co.'s 500 kW prototype at the Fairmont Reservoir fell off.

Why Two Blades?

Designers in Great Britain, Sweden, the United States, and the Netherlands have been uniquely enamored of two blades for decades. The reason? Because two blades are cheaper than three, says University of Massachusetts' professor of engineering Jim Manwell. The lower solidity of two blades to three requires that the rotor spin faster to perform the same amount of work. The higher rotor speed translates into less torque and as a consequence less beefy and, hence, less costly gearboxes.

Manwell, one of the co-authors of the recently published engineering treatise, Wind Energy Explained: Theory, Design and Application adds that two blades also simplifies rotor assembly on the ground. Unlike three-blade rotors which if fully assembled on the ground require two cranes to lift into the vertical position, two-blade rotors can be assembled on the ground in the position they will assume on the nacelle. This could be useful offshore where the rotor could be assembled on deck and raised into position in one lift.

Drawbacks

Manwell, who has operated an old 80-foot (24-meter) diameter two-blade turbine at a test site on Mount Holyoke, quickly notes the technology's well-known disadvantages. "They are noisier," he says, and "some people think they are uglier" than conventional three blade turbines.

Two blade turbines have historically been noisier than three-blade machines because of both their higher tip speeds and because of their greater rotor loading. Early designs that relied on tip brakes for overspeed protection, such as the ESI turbine once operated by UMass' Manwell, were notoriously noisy.

More critically, two-blade rotors present a dynamic or gyroscopic imbalance as the turbine yaws with changes in wind direction. Three blades effectively eliminate this imbalance, as does teetering the two-blade rotor at the hub. With teetering, the two-bladed rotor experiences even fewer cyclic loads than the common three-bladed design say proponents. The benefits of teetering can also be obtained by hinging the blades so they can flap, though control then becomes more complex.

Two blade rotors also capture slightly less energy than three blades. Jack Armstrong, a one-time proponent of two blades while working with Britian's Wind Energy Group, estimated that rotors with three blades can capture about 5% more energy than two-bladed turbines, while encountering less cyclical loads when reorienting the nacelle to changes in wind direction.

Some History

In the early 1940s Palmer Putnam found that the addition of a third blade would add only 2% to total generation, not enough for him in the design of his mammoth Smith-Putnam turbine. He opted for two hinged blades downwind of the tower with dampened coning to minimize flapping.

Meanwhile in Germany, Ulrich Hütter, the "Werner von Braun of wind energy," completed his thesis on optimal wind turbine design. Hütter concluded that high efficiency and low weight were two design priorities necessary to make wind turbines competitive. In 1958 Hütter built a 100-kW prototype embodying his design philosophy. The experimental machine used a rotor 34 meters in diameter comprised of two slender fiberglass blades mounted downwind of the tower on a teetering hub.

Hütter's turbine swept twice the area of the classic Danish wind turbine at Gedser, but less than half that of Putnam's giant. Hütter's turbine in the Schwabian Alps was sleek and aerodynamic in contrast to the clumsy, cluttered appearance of the Gedser machine and Putnam's massive angular design.

Hütter's two-blade turbine fared better than that of Putnam's. His machine operated experimentally from 1956 to 1968 and though Hütter's turbine suffered severe damage on several occasions, he never lost a rotor. Hütter's design became renowned for its high efficiency and his sophisticated approach captivated the dreams of many wind engineers ever since. Though Hütter's design philsophy has been tried repeatedly by many competent engineers for the past 40 years, it has yet to prevail commercially. Some, such as the Carters in the United States and the Wind Energy Group in Britain, have come quite close.

Despite the technology's promise, the market has been cruel to two blades. During their heyday in the mid 1980s, there were some 800 two blade wind turbines operating in California. Carter, ESI, Windtech were prominent names then. But the turbines and their manufacturers fell by the wayside in the great shakeout following the collapse of the California market. With the exception of several Carter turbines in Tehachapi, few of these machines are still in existence. The last derelict Windtech turbines were finally removed from a Tehachapi mountainside at the end of 2002.

With the dominance of three blade designs from Denmark and Germany, two blade technology have remained on the fringe of the wind industry, the domain of Yankees, Kiwis, the Swedes, and the French. "For years people have looked at two-blades suspiciously," says Marco Mingarelli, North American marketing manager for French manufacturer Vergnet.

The fanfare surrounding Henderson's Windflow 500 may mark a turning point. With France's Vergnet, the U.S.'s Wind Turbine Co., and Sweden's Nordic Windpower, two blade designers have a new opportunity to prove themselves--or reconfirm the industry's suspicions.

Vergnet

Until the mid 1990s Vergnet remained a provincial manufacturer of small two-blade wind turbines for islands and isolated networks in the francophone world. The company began a long expansion and modernization drive that culminated in a radical change of orientation. By the end of the decade Vergnet had expanded its product line with the introduction of two blade, downwind turbines, a dramatic reversal of its characteristic two blade upwind rotor with a large offset tail vane. Vergnet flipped the turbine downwind, removing the tail vane. They first developed a 15-meter, 60-kW turbine, then followed that with a 26-meter, 220-kW model. Both have been widely deployed in French overseas territories, notably Guadaloupe where there are about 70 of the 220-kW unit in operation.

Currently, Vergnet is testing a 30-meter, 275-kW version at Chateau Lastours in the south of France. By the end of the year they plan to install 40 production units in Guadalupe for Electricite de France subsidiary SIIF and then develop a 60-Hz version for the Americas. Vergnet expects to install the 60-Hz prototype at Canada's Atlantic Wind Test Site on Prince Edward Island in spring 2004.

One reason for the two-blade, downwind configuration is its light weight. The tilt-up tower is one of the signature features of the Vergnet design says project engineer Jean Michel Fontaine. "Our machine is half the weight of a conventional turbine and can be installed without a crane, ideal for tropical islands and hurricane-prone areas like the Caribbean."

The 60-kW and larger models use a teetered hub. The downwind, passive orientation of these Vergnet models is simpler and more economic, than a yaw-driven upwind machine, says Fontaine. The downwind Vergnet use a small yaw motor for untwisting the pendant cable. Otherwise, yaw is free and the coning of the rotor passively orients the rotor downwind of the tower.

Wind Turbine Co.

Like Vergnet's Mingarelli, Larry Miles feels besieged by the industry's skepticism. To Miles, two-blade designs are like the ugly duckling in H.C. Andersen's fairy tale. The Wind Turbine Co.'s design is a swan trying to break free of prejudice. "We (two-blade proponents) have the same difficulties in the wind industry as the wind industry has in the utility industry," says Miles.

It's been a rocky road since development began in 1995. The failure of the main shaft on the 48-meter, downwind rotor was the most dramatic setback, but not the first. The rotor was slightly damaged in 2002 when it struck the tower. Forensics will determine if the previous incident led to the failure of the 13-foot (4-meter) long main shaft used in Wind Turbine Co.'s novel drive train.

Miles doesn't believe the accident calls into question the fundamental concept of his downwind design. Unlike Vergnet and many previous two-blade turbines, the rotor is not teetered. Instead Wind Turbine Co. relies on two independently flapping blades. And unlike the old Carter turbines, Miles pitches the blades to feather and not toward stall for overspeed control.

Currently with 10 employees, the Wind Turbine Co. has "attempted to turn a government contract into a company," says Miles. To date, development has cost $13-14 million in state, federal, and private funding.

An earlier 33-meter, 250-kW version was operating at NREL's Rocky Flat's test site, until the turbine was taken out of service following the accident in California.

"We were (perceived as) the high-risk company," Miles says about their first government contract. Of the three companies selected (for the government contracts), we're the only one still around. Kenetech is gone and Zond too."

Like Vergnet's design, the Wind Turbine Co.'s rotor orients itself downwind of the tower. But it also incorporates hydraulic dampers that can be used for orientation in what Miles calls "semi-passive yaw."

Prior to the main shaft failure, Miles had hoped to proceed with development of a purpose-built rotor up to 60 meters in diameter. "Our (specific-mass) target for the 750-kW model is 9 kg/m2, about half that for conventional turbines."

Such a lightweight design, about the specific mass of early Carter turbines, enables Miles to suggest that "we could sell the turbine and tower for less than other manufacturers can build their turbines."

In the meantime, the Wind Turbine Co. treads water while it analyzes what failed on the turbine in California and searches for more money. Fortunately for Miles, there are "more people interested funding new turbine technology today than in anytime in the past five years."

Carter Encore

If so, Matt Carter would like them to give him a call. He has a new design that also needs funding. Yes, Matt's the third generation of the storied Carter family to tackle downwind, two-blade wind turbines that his father and grandfather made famous.

No one has ever accused the Carter family of lacking ambition. Carter Wind Energy wants to scale up the 300-kW version from the 1980s to 100 meters in diameter and install them on 200-meter tall tilt-up towers. Matt envisions a prototype 2-MW version followed by a 5-MW commercial unit and subsequently a 10-MW model. Yee ha, Texans think big.

Currently the firm is completing conceptual design, says Matt. There's no hardware yet. They're looking for partners to finance further development. "We believe a two-blade, teetering, or 'soft' design allows us to get the weight down, reducing component cost and subsequently the cost of energy," says Matt.

The rotor will not use delta-3, or pitch-flap coupling. Instead it will use a patent-pending "constant coning" design the company says will lead to smoother operation over varying wind conditions than earlier Carter turbines.

Unlike its predecessors, the new Carter turbine will actively pitch the blades toward stall to control overspeed. Matt explains that the new Carter system is similar to the active stall on some Danish machines. As in the past, Carter will rely on a torsionally flexible spar instead of blade pitch bearings. Though in this version the spar will pass through the hub and into about 1/4 of the blade length.

To avoid the bane of downwind two-blade designs--the whop-whop emitted as the blades pass through the tower shadow--Matt says they've improved blade tip design, will run the rotor at variable speed instead of constant speed, and will move the tip further away from the tower than in earlier Carter designs.

Windflow

Geoff Henderson isn't new to two blades. He was instrumental in developing the Wind Energy Group's MS3, one of the most commercially successful of the two-blade turbine designs. One could argue that the Windflow 500 is a Kiwi version of WEG's MS3.

WEG built about 80 of the MS3 model in the early 1990s: one was installed in California's Altamont Pass, two in Italy, and the remainder in Cornwall and Wales. Garrad Hassan's Peter Simpson, himself a WEG alumnus, says 23 units are still in service, most in Cornwall. Of course WEG itself turned to three blades for its MS4, though it used a highly flexible three-blade downwind rotor, what one wag called a "Carter with three blades".

Key differences with the MS3, says Henderson, are Windflow's use of torque-limiting gearbox Henderson designed for WEG's early MS1 in Cornwall. The torque-limiting gearbox allows the rotor to run at partial variable speed. The MS3 operated at two nearly constant speeds.

After Henderson quantifies the noise emissions from his prototype as part of his contested building permit, and he finds the two blade teetering rotor is "behaving itself" he'll put the machine in full-unattended operation.

Windflow has raised $NZ 3 million in capital to date and hopes to raise another $NZ 5 million. If all goes according to plan Windflow will issue a prospectus for additional shares to begin assembly of a further six turbines.

While an outspoken proponent of two blade rotors, Henderson is equally outspoken about placing the rotor upwind of the tower. "It's silly, says Henderson, about designs by Carter, Wind Turbine Co., and Vergnet. "When are these guys doing to learn about the fatigue loads of running downwind of the tower."

Of course Henderson has to prove that his upwind design can survive the buffeting winds of New Zealand's South Island. "The machine has to work," acknowledges Henderson.

Nordic Windpower

In the early 1990s Swedish engineers wanted to take one more run at jump starting commercial manufacturing of wind turbines across the Kattegat from the successful Danes. In part to distinguish themselves from their Scandinavian competitors known for their three-blade designs, and in part to meet the low cost of energy targets of it's utility partner Vattenfall, Nordic Windpower opted for a lightweight, two-blade upwind rotor driving a planetary gearbox, all features of early Swedish turbines.

In 1992 Nordic Windpower installed a 35-meter, 400 kW turbine incorporating these elements as well as a teetering hub. With European Union funding, Nordic Windpower then went on to build a 54-meter, 1 MW version. Subsequently, they installed two more units at Nässudden on Gotland. Then early this year they installed a 59-meter, low-wind version on a 70-meter tall tower in the province of Halland.

Nordic Windpower's plans are unknown, the company failed to return WindStat's phone calls.

Dutch Drop Two Blades

Outside the Americans, the Dutch have probably fielded the most two-blade turbines. Several Dutch manufacturers were known for their two-blade upwind designs, notably Nedwind and Lagerwey. There were also other, lesser known companies such as Newinco.

Wind pioneer Herman Drees proudly reports that the 20 Nedwind 40s in the San Gorgonio Pass are now in their ninth year of operation. The 40.1-meter, 500-kW turbine uses two variable pitch blades upwind of a conventional tubular tower, making them among the largest two-blade wind turbine designs ever commercially produced.

Nedwind also fielded nearly two dozen 55-meter, 1-MW versions in the Netherlands. Both models used full-span pitch control with active stall that Nedwind developed in the early 1990s. Despite their size, the rotors were not teetered, necessitating a heavier hub than on a comparable three-blade turbine, says Drees. "I have no experience with teetered hubs," he adds, but with teetering "you must pay very close attention to your system's dynamics." With teetering, "you win some places and you loose in others."

The Nedwind 40s have "generally met our performance expectations," Drees says. "We knew early on that we would have soiling issues, and we have to wash the blades often." That's not a problem unique to two blades.

Innovative designer Henk Lagerwey switched to two upwind blades on a teetered hub early in his development cycle. During the late 1980s and early 1990s, Dutch farmers installed hundreds of his 18-meter, 18-kW model that used a novel pitch-to-stall hub coupled with a variable speed drive train. Lagerwey was one of the first to use variable speed drive with power electronics on a commercial wind turbine. Subsequently, he introduced a 25-meter, and later a 30-meter version of his ungainly turbine.

Not long after introduction of the 250-kW model, Lagerway abandoned two blades for a three-blade upwind rotor directly driving a 750-kW ring generator, several dozen of which have been installed in the Netherlands and Japan.

Researchers at ECN continued development of two-blade technology for more than two decades at their test field near Petten in Noord Holland. Their 25-meter test turbine was at one time among the largest turbines in the world. It's last incarnation, the FlexHAWT, was removed five years ago says senior ECN project engineer Bernard Bulder.

The flexible two-blade rotor was difficult to control. "There were stability problems, and cracks (developed) in the blade roots," says Bulder. We "still believe two-bladed machines might be useful for offshore," where the slightly lower energy capture of the two-blade rotor may be offset by installation and servicing advantageshe says. The greater noise and lower aesthetic acceptability of two-blades may be less disadvantageous offshore as well.

Can They Do It?

Can two-blade turbines survive in the marketplace? Possibly, say designers, but they first have to catch up with three-blade designs that continue to grow bigger and more reliable. And as turbines become bigger the cost of market entry becomes astronomical.

ECN's Bulder's wonders if today the risk may be too great to change to two-blades. Major manufacturers stick with the technology they know, he says. "We have a proven technology with three-blades," but equally as important "we know the problems" with three blades. Perhaps, wonders UMass' Manwell, we've learned enough now about making three-blade wind turbines work that we can now successfully tackle two-blade machines.

"If you can make them (two-blades) work, they're obviously less expensive, says Bob Lynette, onetime developer of the AWT-26, a 275-kW, 1990s derivative of the ESI-80. "If you can design it to deal with the loads" they stand a chance, Lynette says. But development "can't be done on a shoestring, it must be well funded." Developing two blade rotors today is "a high risk program and that's the role of the government".

What would Lynette, who poured $5-6 million ("that wasn't all government money by any means"), do differently today? He would only undertake the project if there was enough money to do it right.

Lynette's vision, like that of the Carters, were thousands of such turbines on the sparsely populated plains--the Buffalo Commons region--of North America where there would be little visual or noise limitations. That's where a 2-blader can shine," Lynette says.